I'm sure that before my surgery I would've passed a psych exam no problem, but I had a lousy result and nine months post-op I'm emotionally messed up. Clinical depression and certainly symptoms of BDD. Of course in my case there are other external factors that have contributed to this, but I think this kind of thing really has the power to shake someone up. Lots of people, maybe most, have a good result and can move on pretty quickly, but emotional trauma, obsessive behaviour, major depression, etc., which all impair day-to-day functioning, are risks that surgery candidates should be aware of, especially if looks are an important part of the decision to undergo surgery (and I think they almost always are). A poor result can mean a lot more than just a poor result, if you know what I mean.
I haven't had surgery yet but this is one of my biggest fears. You do the surgery in hopes of a better quality of life but you end up with a much worse one due to the emotional/psychological effects.